This invention relates to a flow control device for use in construction equipments or the like, particularly to a flow control device capable of providing two classes of oil flow.
A conventional flow control device will be described with reference to FIGS. 2 and 3. As shown in FIG. 2, in the conventional flow control device, a pump 1 is communicated with a hydraulic motor 4 through a flow dividing valve 2 and a control valve 3. The flow dividing valve 2 is adopted for controlling the flow of oil supplied to the control valve 3. The control valve 3 is adopted for controlling the direction and flow of oil supplied to the hydraulic motor 4.
The flow dividing valve 2 has a first position 2A in which the whole flow of oil discharged from the pump 1 is supplied to the control valve 3, and a second position 2B in which a portion of oil discharged from the pump 1 is supplied to the control valve 3 and the remaining portion is discharged to a tank T0. In addition, the flow dividing valve 2 has a pilot port 2C to which a pilot line 2F is connected, and springs 2D and 2F. The pilot port 2C is communicated with the tank T0 through the pilot line 2F having a selector valve 6. A portion of oil flowed into the flow dividing valve 2 is supplied to the pilot line 2F as pilot oil, so that when the pilot line 2F is disconnected from the tank T0 by the selector valve 6, the flow dividing valve 2 is shifted from the first control position 2A to the second position 2B, when the pilot line 2F is communicated with the tank T0 by the selector valve 6, the flow dividing valve 2 is shifted from the second position 2B to the first control position 2A.
The control valve 3 has a neutral position 3A in which both a line between the hydraulic motor 4 and the pump 1 and a line between the hydraulic motor 4 and the tank T0 are closed, and a bleed-off control passage 3F is opened, a first control position 3B in which both the line between the hydraulic motor 4 and the pump 1 and the line between the hydraulic motor 4 and the tank T0 are opened, and the bleed-off control passage 3F is throttled, and a second control position 3C in which both the line between the hydraulic motor 4 and the pump 1 and the line between the hydraulic motor 4 and the tank T0 are opened, and the bleed-off control passage 3F is closed.
Indicated at 7 is a bleed-off line for communicating the bleed-off control passage 3F with the tank T0. Indicated at 3B' and 3C' are reverse first and second control positions having the same functions as the first and second control positions 3B and 3C but the reverse flow directions to the first and second control positions 3B and 3C.
In the above-mentioned conventional construction, when the pilot line 2F is closed by the selector valve 6, the flow dividing valve 2 is shifted to the second position 2B, so that a portion of oil discharged from the pump 1 flows to the tank T0 and the remaining portion flows to the control valve 3.
In this state where the flow dividing valve 2 is in the second position 2B, the control valve 3 is shifted to the first control position 3B from the neutral position, the flow of oil supplied to the hydraulic motor 4 is controlled by throttling the bleed-off control passage 3F. This flow control corresponds to a portion J1 of a performance curve J shown in FIG. 3.
Further, when the control valve 3 is shifted to the second control position 3C, the whole flow of oil supplied from the flow dividing valve 2 is supplied to the hydraulic motor 4. This flow control corresponds to a portion J2 of the performance curve J.
Also, in another state where the flow dividing valve 2 is shifted to the first control position 2A by opening the pilot line 2F, and the whole of oil discharged from the pump 1 flows to the control valve 3, when the control valve 3 is shifted to the first control position 3B from the neutral position, similarly, the flow of oil supplied to the hydraulic motor 4 is controlled by throttling the bleed-off control passage 3F. This flow control corresponds to a portion K1 of a performance curve K shown in FIG. 3. Further, when the control valve 3 is shifted to the second control position 3C, similarly, the whole flow of oil supplied from the flow dividing valve 2 is supplied to the hydraulic motor 4. This flow control corresponds to a portion K2 of the performance curve K.
Accordingly, the speed of the hydraulic motor 4 is controlled by the control valve 3 in accordance with the portions J1 and J2 of the performance curve J, and the portions K1 and K2 of the performance curve K.
The bleed-off control passage 3F has a controllable range between a full-opening position S0 and a full-throttling position S1. When the bleed-off control passage 3F is in the full-opening position S0, the bleed-off control passage 3F is fully opened so that the whole flow of oil discharged from the flow dividing valve 2 which is in the first position 2A is permitted to flow to the tank T0 through the bleed-off line 7 and no oil is supplied to the hydraulic motor 4. On the other hand, when the bleed-off control passage 3F is in the full-throttling position S1, the bleed-off control passage 3F is fully throttled so that the whole flow of oil discharged from the flow dividing valve 2 which is in the first position 2A is permitted to flow to the hydraulic motor 4 and no oil is flowed to the tank T0 through the bleed-off line 7.
In the case that the flow dividing valve 2 is shifted to the second position to reduce the flow of oil supplied to the control valve 3, however, the oil discharged from the flow dividing valve 2 does not start flowing to the hydraulic motor 4 until the bleed-off control passage 3F is throttled by a partial throttling position S0'. This is because of the fact: When the flow dividing vavle 2 is in the second position 2B, a reduced flow of oil is supplied to the control valve 3. Therefore, the oil cannot have pressure enough to overcome an inertial resistance of the hydraulic motor 4 until the bleed-off control passage 3F is throttled by the partial throttling position S0'. Consequently, the slope of the portion K1 is identical with that of the portion J1.
The controllable range of the bleed-off control passage 3F of the control valve 3 is reduced when the flow dividing valve 2 is in the second position 2B. Accordingly, it will be seen that the maximum speed of the hydraulic motor 4 is changed by shifting the flow dividing valve 2, but it is impossible to finely control the speed of the hydraulic motor 4 at the reduced flow of oil.